TEHRAN (Reuters) - Defeated candidate Mirhossein Mousavi demanded on Sunday that Iran's presidential election be annulled and urged more protests, while tens of thousands of people hailed the victory of the hardline Mahmoud Ahmadinejad.

Mousavi's supporters again took to the streets after violence on Saturday, clashing with police in protests that have underscored political rifts exposed by Friday's disputed vote.

In a statement on his website, Mousavi said he had formally asked the Guardian Council, a legislative body, to cancel the election result.

Mousavi's supporters handed out leaflets calling for a rally in Tehran on Monday afternoon. After dusk some took to the rooftops across the city calling out "Allah Akbar" (God is greatest), an echo of tactics by protesters in the 1979 Islamic revolution.

The unrest that has rocked Tehran and other cities since results were declared on Saturday is the sharpest expression of discontent against the Islamic Republic's leadership for years.

The election result has disconcerted Western powers trying to induce the world's fifth biggest oil exporter to curb its nuclear program. U.S. President Barack Obama had urged Iran's leadership "to unclench its fist" for a new start in ties.

U.S. Vice President Joe Biden cast doubt on the election result but said Washington was reserving its position for now.

"It sure looks like the way they're suppressing speech, the way they're suppressing crowds, the way in which people are being treated, that there's some real doubt," he told NBC's "Meet the Press" when asked if Ahmadinejad had won the vote.

SEA OF FLAGS

Ahmadinejad appeared amid a sea of red, white and green Iranian flags waved by partisans thronging Tehran's Vali-e Asr square, some perched on rooftops or cars, to applaud the victory he achieved with a surprising 63 percent of the vote.

"Some ... say the vote is disrupted, there has been a fraud. Where are the irregularities in the election?" he said in a speech that the crowd punctuated with roars of approval.

"Some people want democracy only for their own sake. Some want elections, freedom, a sound election. They recognize it only as long as the result favors them," he declared.

Tarverdi Chegine, a 35-year-old government employee, told Reuters: "We have a very brave president. I love him."

He said anti-Ahmadinejad protesters were not true Iranians. "They belong to the West. They belong to Bush. We are anti-Bush."

After the rally, witnesses said Ahmadinejad and Mousavi supporters clashed on a main Tehran street. A Reuters reporter saw fires and broken glass on the street, people throwing stones, and riot police on motorbikes. One policeman was beating people on the pavement with a rubber truncheon.

About 2,000 students at Tehran University, some with Mousavi posters, others covering their faces with bandanas, chanted anti-government slogans and taunted riot police across the road outside. Some threw stones at police when they chased protesters who had tried to gather outside the university gates.

Abdul Reza, 26, standing behind the gates and watching as police charged the crowd outside, said: "Mousavi is the real president of Iran. Ahmadinejad did not win the election."

Speaking at a news conference Ahmadinejad described the election as "clean and healthy" and dismissed complaints by defeated candidates as sour grapes.

He consigned Iran's nuclear dispute to the past, signaling no nuclear policy change in his second term, and warned that any country that attacked his own would regret it. "Who dares to attack Iran? Who even dares to think about it?" he asked.

Iran's refusal to halt nuclear work the West suspects is aimed at making bombs, a charge Tehran denies, has sparked talk of possible U.S. or Israeli strikes on its nuclear sites.

FRENCH CONCERN

France signaled its concern over what was happening in Iran, which Henri Guaino, a top adviser to President Nicolas Sarkozy, said "is clearly not good news for anyone, neither for the Iranians nor for peace and stability in the world."

French Foreign Minister Bernard Kouchner said repression of opponents was closing off dialogue. "Brutality and never-ending military development will not bring any solutions," he said.

German Foreign Minister Frank-Walter Steinmeier condemned any action against demonstrators and said Germany was watching with "great concern."

Police have detained over 100 reformers, including a brother of former President Mohammad Khatami, a leading reformer said. A police official denied Khatami's brother had been arrested.

Interior Ministry officials have rejected accusations of election fraud and Supreme Leader Ayatollah Ali Khamenei, Iran's top authority, has called on Iranians to back their president.

A senior Western diplomat in Tehran said he believed the authorities would soon subdue the street unrest, but said Ahmadinejad's re-election battle had exposed a polarizing power struggle between radicals and moderate conservatives which could affect the Islamic Republic's long-term stability.

"There is turbulence in the whole system," he added.

A spokesman for Mousavi said his newspaper, Kalameh-ye Sabz, and its website had been shut down. Mobile telephone text services have also been interrupted in Tehran for several days, and the British Broadcasting Corporation said Iran was using "heavy electronic jamming" to interrupt its widely watched BBC Persian television service.